At present, biomedicine, environmental monitoring, food safety, even national defense and other fields all require high-sensitivity sensors urgently. Optics-based sensors, due to their high sensitivity, wide application range, easy operation, and abundant function, have been developed greatly. Label-free optical detection technology, which is to sense by perceiving change in the refractive index of samples to be tested, without requiring any processing on the samples, so that the samples in a natural state may be detected in real time and quantitatively at low cost, has been widely used. Generally, such an optical label-free sensor runs in both the visible band and the near-infrared band. In recent years, terahertz band sensors have attracted people's attention. The terahertz frequency (0.1 THz to 10 THz) is between infrared and microwave, and many bio-macromolecules have a vibration frequency within the terahertz band and a characteristic absorption peak. Therefore, the terahertz sensing shows better identification capacity and higher sensitivity. Additionally, specifically the optical sensing of liquid samples, in order to realize sensing detection with fewer samples and to more effectively control reaction and separation between micro samples and the like, the micro-fluidic channel technology is developed rapidly. Currently, the combination of the micro-fluidic channel with the terahertz technology becomes one tendency of the optical label-free sensing technology.
A terahertz micro-fluidic biosensor based on micro-strip transmission lines was reported in the Applied Physics Letters, Vol. 93, P182904, 2008. This sensor realizes sensing, by detecting change in optical transmission properties resulted from the coupling between evanescent waves on the surface of the micro-strip lines and the liquid sample in the micro-fluidic channel. A terahertz micro-fluidic sensor based on planar waveguide resonators was reported in the Applied Physics Letters, Vol. 95, P171113, 2009. This sensor improves the sensitivity of detection by using the effect of resonators. Biosensing by a perfect absorber made of metamaterial was reported in the Nano Letters, Vol. 10, P2342, 2010. Enhancement of sensitivity of a terahertz detector by the near-field localization properties of a metal micro-nano antenna structure was reported in the Optics Express, Vol. 20, P5052, 2012. A sensor based on metamaterial was proposed in the Applied Physics Letters, Vol. 100, P221101, 2012. The aforementioned technologies all realize sensing, based on superposition between samples to be tested and the near-field evanescent waves of a resonant structure, by measuring change in the evanescent waves along with change in the refractive index of the samples to be tested. Therefore, the sensing is limited to the degree of superposition between the evanescent waves and the liquid to be tested, and the improvement of sensitivity is limited.